The present invention relates to a photodetector, like for example a photodiode, an image sensor having a plurality of photodetectors and corresponding methods for manufacturing.
In the past years, CMOS image sensors have acquired a large share of the market for general photo sensor technology applications. If, however, standard CMOS image sensors are used with applications which need the detection of light with a very small spectral bandwidth or (as a boundary case) the detection of monochromatic light (as it is conventional with applications of 3D runtime image generation or with many applications with laser light irradiation), specific effects become important which are negligible with standard photo sensor technology applications with a broad spectrum illumination.
A specific effect which may be observed is that for a given light wavelength to be detected strong fluctuation of sensitivity of the photodiode exists (wherein several of the same form an array). This observed fluctuation of sensitivity depends on small process-related fluctuations of the dielectric stack on the photodiodes and leads to great wafer-to-wafer and chip-to-chip fluctuations of spectral sensitivity. In a similar way, this also relates to different photodiodes on one single chip and thus influences the pixel response non-uniformity.
If, for example, conventional CMOS image sensors or CMOS photodiodes with narrow-band light are irradiated effects occur which are negligible with normal applications. Among others, with a narrow-band illumination an oscillation of the spectral sensitivity with a strong dependency on small variations of the wavelength and on small variations of the layer thicknesses of the individual dielectric layers occurs which are deposited onto the silicon in CMOS processes and insulate metal traces from each other. The interference of partial beams of the individual material interfaces leads to oscillations of sensitivity as the irradiation power which reaches into the silicon is modulated (see FIG. 1). The entirety of these layers is also referred to as an optical stack, is typically about 5 μm thick and conventionally consists of silicon oxide or silicon nitride and possibly further layers like color filters.
With broadband applications, in the spectrum of the incident light wavelengths are contained which are in different ranges of oscillation, so that an averaging takes place and no problems exist. With applications with a virtually monochromatic illumination (e.g. by a laser) the insecurities of spectral sensitivity by process-related layer thickness fluctuations have to be considered, however. If, as a concrete example, monochromatic light with a wavelength of approximately 750 nm is used as a basis and a layer stack of 4.4 to 6.4 μm silicon oxide and 650 to 850 nm silicon nitride (these are realistic layer thickness fluctuations), then this is accompanied by a quantum efficiency between 0.37 and 0.81 (see FIG. 4a). The difference of maximum and minimum quantum efficiency in this case is 0.43. For many applications like, e.g. the 3D time of flight imaging, triangulation sensors and spectroscopy, these insecurities of spectral sensitivity are not tolerable, as they have an effect on insecurities of the measurement value.
One possible solution to the problem is a calibration. In this step, after manufacturing each image sensor is illuminated with a known illumination or irradiation strength and the resulting current is determined. From this recalculation, the sensitivity of each individual photodiode may be determined, which is subsequently either directly stored in the image sensor or in the camera system as calibration data or has to be considered in data processing later on.
Another theoretical way may be anti-reflex coatings (ARC). Here, a difference has to be made between three different ARC terms. In connection with commercial CMOS image sensors, frequently an SiN layer of <100 nm is deposited on a very thin SiO2 layer (approximately 10 nm) above the photoactive areas. As the refractive index of SiN is between that of Si and SiO2, and the SiO2 layer between SiN and Si is very small, the reflection is reduced. In general, an ARC is a multilayer sheet of different materials which suppress reflection by an advantageous utilization of interferences. For this method, the refractive indices of the used layers have to fulfill certain relations so that an optimum result is acquired. As a special case, this multilayer sheet may be manufactured from SiO2 and SiN, which does not lead to optimum results, however, as the requirements to the refractive index may not be exactly fulfilled.
The first possibility has the advantage that it may do without new materials. SiN is well known and thus exists in many CMOS processes (see for example Furumiya, M.; Ohkubo, H.; Muramatsu, Y.; Kurosawa, S.; Okamoto, F.; Fujimoto, Y. & Nakashiba, Y High-sensitivity and no-crosstalk pixel technology for embedded CMOS image sensor Electron Devices, IEEE Transactions on, 2001, 48, 2221-2227). The integration of a correspondingly thick layer in photoactive areas is needed, however. Simulations in which different layer thickness combinations were examined do show a reduction of reflection and thus also of the process-dependent sensitivity fluctuations, but the reduction which may be acquired is substantially lower than desired.
If an ARC layer stack with materials is considered which are not normally used in CMOS processes, many new challenges result for example with regard to adherence, patternability stability and integrability into CMOS processes. A special challenge here is maintaining the layer thickness specifications to be able to stick to interference conditions. Scientific publications on this topic are rare (see Vaillant, J.; Grand, G.; Lee, Y.; Raby, J.; Cazaux, Y; Henrion, Y. & Hibon, V. (Eds.) High Performance UV Anti-Reflection Coating for Backthinned CCD and CMOS Image Sensors, ICSO 2010). A further disadvantage of both methods is that the requirements to fluctuations of the layer thickness are substantially higher than with this invention.